第一步，读懂材料，抓住主旨。
首先要注意材料的特点：单则材料要从整体上把握其主旨；多则材料应弄清它们的异同，从而把握中心；比喻性材料要理解其比喻意义；含哲理性寓言材料，要把握其寓意。所谓抓住主旨（即材料的意想倾向和感情倾向），
其方法是：一要抓住材料中关键句子或暗示性语言；二要注意对比材料划分层次，归纳层意，综合分析材料的基本观点；三要用正确的观点和对正反材料作出正确的判断。
例如：
阅读下面的材料，根据要求写一篇不少于800字的文章。
乌龟和兔子赛跑，因为兔子骄傲，乌龟出人意料地赢得了比赛。蝴蝶知道此事后，也想和乌龟比赛。乌龟考虑了一下，便接受了蝴蝶的挑战，但他提了一个要求：“我只会爬行，你还会飞。我们就比60米吧，20米钻圈，20米河流和20米花丛。”蝴蝶同意了。比赛开始，蝴蝶早就钻过了圈，飞过了河，可当来到花丛时，她看到了那漂亮的花朵，嗅到了花蜜的香甜，还见到了美丽的同伴。看看此时乌龟连圈都没钻过，蝴蝶放心了，她在花丛里玩得越来越开心，竟然忘记了比赛的事情。万万没有想到，乌龟已经钻过了圈，穿过了花丛，悄悄地到达了终点。
对某些人来说，钻圈、过河流很难，可对某些人来说，花丛才是最难逾越的。
要求选准角度，明确立意，自选文体，自拟标题；不要脱离材料内容及含义的范围作文，不要套作，不得抄袭
这则寓言故事的内涵是十分丰富的。要准确把握材料的含义，可以从以下几个方面来思考：
1．为什么乌龟能够赢得比赛。
2．蝴蝶为什么如此流连花丛。
3．联系现实人生，钻圈、过河指什么？花丛又是什么？
4．蝴蝶为什么能够迅速钻过圈、飞过河，却过不了花丛？
指导：
据此可以立意：我们的人生应该有个大目标、大方向。乌龟之所以能够赢得比赛，就是应为他能时
刻牢记自己的大目标。而蝴蝶之所以半途而废，则是因为没有能够摆脱一些东西的诱惑，由此迷失了方向。
写作时，可以从整体上思索，可以从细微处发掘。值得思考的有很多。一定要写出个人的认识。
第二步，看清材料作文要求，抓住原材料的主旨提炼出一个正确的主题（即观点），确立中心（即立论）。
主旨单一的材料据材料立意即可，例如我们在考试中遇到的第二个作文（立意时应从“团结”入手）。蕴涵丰富的材料，可从多角度立意，即发散思维多向立意（例如第一个作文可以分别从“分享”“奉献”的角度入手）。一般说来，一则材料至少可从肯定与否定两个角度审视：“一事多人”的材料，有几个“人”往往就有几个审视角度；“一事多因”的材料，有几个“因”往往就有几个审视角度。一个事件，针对其背景、写作目的的不同，就产生了几个不同的审视角度。在这一环节，可考虑从以下三个方向发散：①横向发散：即由材料的基本主题展开引申类比联想，由此及彼，比较求同，看哪些事物或现象是有类似或相关的属性，或者这个道理可以推及到哪些事物或现象上。②纵向发散：即研读材料所表现的社会现象或事实，并由表及里，追根溯源，寻求事物发展变化的因果关系，把握其所反映的普遍而深刻的道理。③逆向发散：即由正而反，从事物的相反方向思考，多问几个如果不这样将会如何。有了以上角度开掘出来的主题后就要选最佳的角度和立意来写。
第三步，有的同学将这次考试中的作文写成了议论文（我们讽刺这类文章为“四不像”文章），不过想写成议论文也未尝不可，我们上期的课本就选了很多议论文，不过你要掌握一些方法------运用材料合理入文，引议联结构思谋篇（根据曹组长讲话整理）。①“引”，分析材料，提出观点或中心论点。②“议”，承上启下，展开议论（先写一个过渡段或过渡句，再写本论）。一般来说，应先正面论述中心论点，揭示正面道理，然后举正面事例论证“为什么是这样，有何好处”之后，最好再能从反面事例加以论证“为什么不可以这样”，举反面事例论证。③“联”，联系实际，深化中心（主要是社会或个人的实际，从生活中选取典型材料进一步论证中心论点）。④“结”，总结照应，解决问题（可以照应材料或开头，起点题作用或总结、升华、发号召等）。这个公式可用，但千万不要搞成新八股文。材料作文是根据既定材料，对材料进行分析、提炼，从而得出一定的看法和观点的一种作文形式。它既有命题作文的限制性，又有自拟题目作文的自由性；既能考查学生按要求作文的能力，又给学生提供了充分显示写作能力的广阔空间。由于材料作文所具有的灵活性、丰富性、与时俱进性，这类作文命题形式备受命题作文老师的青睐。
综观近年全国各地中考作文题都少不了材料作文。
我们考试中遇到的第一篇作文就是河南省2009年中考作文二选一的第2篇
材料作文所提供的材料的来源和形式是多种多样的，或是寓言故事、历史事实，或是诗歌、漫画、名人名言，或是最近发生的事件、报刊上刊登的新闻等等。这些材料有时是单一材料，有时则是一组材料。材料作文包含的内容很广泛，根据题目的要求，可以写成记叙、说明、议论等各种文体来写作，甚至，有的要求写一篇文章的读后感，有的要求根据材料自选角度写议论文，有的要求根据图画、画表写一段说明文字，还有的要求以提供的材料为开头续写，或根据材料扩写、缩写。
三、写好材料作文立意也是一个关键，作文审题立意要准。由于材料本身是松散的、不紧凑的，它的意思是由阅读者本人领会的，所以造成理解上的多样性。这种多样性，只要符合材料的题旨、命题者的意图，是允许存在的。但问题在于许多考生不能准确把握材料所传达的信息。要想准确把握题旨必须对材料加以提炼整合，即把材料中那些共通的精华的东西提炼出来，而不能随心所欲地判定或凭主观意念硬贴上去。一般性材料的题旨和意图是比较容易分辨的，而寓意性材料的题旨和意图却不那么容易分辨，因为它们的题旨和意图足隐含的、不外露的，只有更深一层地理解分析它的意义，才能正确地把握题旨和意图。好多同学将此次考试作文拟题为《分享》《团结就是力量》题目过大、过于平淡，让我阅卷时找不到你文章的中心，找不到出彩的地方，如何避免这种情况呢？
认真阅读材料，吃透材料是写好材料作文关键的一步。如果没有准确把握材料的主旨，就会写出偏题、跑题的作文。把握文章的主旨有以下几种方法。
1、注意材料的标题。有的文章标题就是文章的主题。
2、注意材料的开头和结尾。有的文章在开头或结尾就点出了文章的中心，尤其是文章的结尾。
3、注意材料里面抒情、议论性的句子，出题者往往在文章里借助抒情、议论性的句子点出文章的中心。。
4、注意材料后面的要求或提示语。例如：一篇材料作文在原文后面，它提出了这样的要求，“读了材料我们很受启发，请就其中的一个方面（如成功与失败、诱惑、机遇------）自拟题目，自选文体，写一篇文章，不少于600字。”熟读材料后再看提示，不难发现，材料中的三层含义“成功与失败”“诱惑”“机遇”是写作的三个范围，可据其中的一个方面立意写作文。
同时要把握材料作文的审题思路。审题是决定作文考试成败的关键。由于中考作文应试时间少，审题不可能花费太多的时间，因此，对文字表述较为复杂的材料必须集中思想，尽快地把握审入?选择什么立意?重点采用什么表现方法?如何体现文章构思?等等，之所以这样，是因为作文的材料往往可以从不同的角度来落笔，确定不同的立意，选定不同的写作方法，寻求不同的构思。其次还要注意文题的其他要求。作文题中常有一句“除诗歌之外，文体不限”的提示，对此理解必须注意两点：一，所谓“不限”，一般是指构思时对文体的选择余地较大，但一旦选定某种文体进入具体写作过程时，则应遵循某种文体的基本格局，不能写成议论文不像议论文，记叙文不像记叙文的“四不像”的文章。最后，要认真阅读，把握材料作文的主旨。有些同学考试中作文的分很高，有些得分却很低，为什么得分低呢？看看我下面的讲解你就知道了：
四、材料作文的写作“四忌”。
1、忌抛开材料，“另起炉灶”。材料作文和话题作文尽管都有材料，但二者对材料的处理有所不同。话题作文强调的是相关，材料可用可不用；材料作文所提供的材料是写作的依据，包孕着文章的主旨，材料在文中必须出现。不少考生写惯了话题作文，导致忽视了材料作文中的材料，以致于“另起炉灶”。例如有些同学就拟题为《叛逆也需要轨道》简直让人不知所云，怎么会的高分呢？
2、忌不按指令写作。材料作文的试题，一般包括两上部分，一部分是材料，另一部分是写作要求。写作要求或以提示语出现，或另列出“要求”、“注意”。审题时，不仅要吃透材料，还必须看清写作的具体要求，按照试题的指令写作。切忌不看要求，或未看清要求，即下笔千言，却离题万里。
3、忌观点提炼不准确。材料作文的文章主旨是从材料中得来的，考生如果在阅读材料时提炼的观点背离了材料的内涵，那么将直接导致该篇作文的失败，即是“皮之不存毛将焉附？”。
4、忌漠视材料中的关键句。作为材料作文，每篇材料都有关键句，只不过有的不明显而已。关键句在文段中所起的作用或画龙点睛或提纲挈领，一些考生过分的强调整篇材料主旨而忽视了关键句，导致写出的文章很难出彩。第一，要明确材料作文和话题作文的区别。材料作文和话题作文的区别在于话题作文只提供一个话题，材料只引出话题，与写作无关。而材料作文则提供一定的材料。如一首诗、一则寓言、一个或几个典故、一个故事、几则名言警句、一幅或几幅画，是对现实生活中某种现象的介绍、社会某种看法的概述、一个特定情境的设计等等，要求基于所供的材料，根据题目要求和规定思路导向进行写作。
最后我想说的是，我们有些同学在考试时为了抢时间，未及对材料细思慎辨，就匆匆落笔；甚至曲解了所提供的材料，其结果自然是“欲速则不达”，这就难免留下遗憾了。
考试中有些同学考试中作文的分很低，甚至想找我理论。我觉得你大可不必烦恼，看看我在此次考试中发现的一些问题你就会明白为什么你的得分较低了：
一、写作文的过程中有些同学未经深思出现了跑题偏题现象。
“文章成败由立意”，跑题偏题是考场作文最要命的“硬伤”。审题经常出现的毛病有两种，一种是完全抛开题目(话题、材料)、另起炉灶而造成跑题，另一种是为了标新立异、另辟蹊径而造成偏题。审题要把握“三义”。一是要看懂字面义：准确地理解题目(话题、材料)的字面义(尤其是关键词语)是领悟题旨的第一步;材料作文和话题作文，题目的文字一般比较多，这就更需要仔细阅读，正确理解，全面把握。二是要吃透引申义：引申义是蕴涵在题目里面的，需要考生具有一定的认识水平和分析能力才能感知;题目的引申义，还包括它的比喻义和象征义。三是要领会提示义：有些题目附有提示语，提示语是命题人对考生的一种暗示，是为考生审题立意、打开思路铺设的台阶;提示语不论长短，考生都要高度重视，仔细阅读，认真揣摩，积极地从中获得写作的启示。审清题意之后，考生还要学会在行文中不断地扣题和点题，做到“一步三回头”、“回眸一笑百媚生”。
二、个别同学文体不分
材料作文一般要求“三自”——角度自选、文体自选和题目自拟。文体的开放性给考生创设了扬长避短的条件，同学们可充分发挥自己的优势。但是不少学生误以为“淡化文体”就是不要文体，致使所写的文章不伦不类：写议论文，不像议论文，不知道提出问题、分析问题、解决问题;写记叙文，人、地、事、时、物交代不明不白;写散文，线索不明晰，形散神也散。掐头去尾就没东西了，这样非驴非马的“四不像”作文只能在三类卷以下。同时不要贸然去写诗歌。建议考生根据自己的喜好，扬我所长。
三、立题平淡
标题是文章的眼睛。好的标题常常能起到先声夺人、在考试中，作文没有标题原则上要扣一定的分数。而事实上没有标题的作文不是一篇完整的文章，它会影响主题的表达，阅卷老师会认为你缺少了最基本的东西，在心理上会产生一种距离，也就在不知不觉中加大了扣分，考生可能会因此失去更多的分数。建议你平常养成先写标题后写正文的习惯，而且标题要写在正中间位置，同时写完文章后要仔细看一遍，这也是避免出现这类“硬伤”的有效方法。
四、有些学生的作文，开头排比很长很长，结尾同样很长很长，甚至还有后记。
这种“硬伤”表现在背离了“凤头豹尾”的基本要求，开篇缓慢，久久不能点出主旨;结尾拖沓冗长，无病呻吟。无论是开头还是结尾，都应该简洁，一般说来，每部分不得超过文章总字数的十分之一，才能充分保障“猪肚”的丰富性。建议文章开头开门见山，不蔓不枝，用刘勰的话说是“首唱荣华”，借用俗语就是“一句请出佛祖来”。结尾，或画龙点睛，收束全文;或照应开头，结构严谨;或引用诗文，点染升华;或抒情写意，余味无穷;或设问反问，启人心智。
五、引用的实例或诗句不准确
在阅卷时有些学生张冠李戴、贻笑大方。建议：一是加强平时的积累，尽可能避免犯常识性错误;二是尽可能避免如“我在电视上看到过这么一个故事或是曾经听过这样的一个故事或是那是小学三年级的事了……”等大篇幅的引用;三是如果对引用的诗文词句记不清，考生可以转述，即使出错，也一定会得到阅卷老师的谅解。
六、有些同学言辞偏激胡乱批评
青春期的你们容易意气用事，思想偏激，说“过头话”，语气武断。在考试写作文的时候，建议你以平和、辩证的心态来写，这样写出来的东西就会显得大气，给人波澜不惊的感觉，老师看了也会觉得舒服，千万要避免剑走偏锋，观点偏激。那样会让你的作文显得幼稚可笑。
七、卷面脏乱
卷面是你给阅卷老师的“见面礼”。有些学生由于平时养成了马虎潦草的坏习惯，作文卷面总是勾画涂改得脏兮兮的，不堪入目：有的字迹潦草，难以辨认;有的胡涂乱抹，面目全非。从阅卷的实际情况来看，书写规范、端正、美观，卷面整洁的作文，一开始就会赢得阅卷老师的青睐，阅卷老师对一些非原则性的问题也不会计较。反之，尽管文章观点鲜明，情节生动，语言流畅，文笔优美，但让人看着费劲，最终所扣的分数绝不止规定的书写分。另外，作文时不要用多色笔，字体不宜过大或过小，标点符号要占格。
八、不该字数超常
所谓字数超常，就是说字数过少或过多。有的学生写作时一发而不可收，洋洋洒洒上千言，方格内盛不下，边幅上都写满了，给人一种臃肿庞杂的感觉。解决这个问题的办法就是：充分利用好试卷所给的空间即可，写得不可过多或过少。
九、有些学生认为只要写字了老师就会给分，以至于剩下很少的时间去仓促成文
语文考试，有的时候考生因为精神紧张，往往忽视了对时间的合理安排，致使占“半壁江山”的作文未能写完，成为“残篇”。建议你平时作文按时完成，模拟考场考作文严格控制在40分钟之内完成即可。
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as a living thing, and is often called the building block of life.[1] Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an unfertilized ostrich egg cell.[2]
In 1835 before the final cell theory was developed, Jan Evangelista Purkyně observed small "granules" while looking at the plant tissue through a microscope. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.[3]
The word cell comes from the Latin cellula, meaning, a small room. The descriptive term for the smallest living biological structure was coined by Robert Hooke in a book he published in 1665 when he compared the cork cells he saw through his microscope to the small rooms monks lived in.[4]
[edit] General principles
Mouse cells grown in a culture dish. These cells grow in large clumps, but each individual cell is about 10 micrometres acrossEach cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.
All cells have several different abilities:[5]
Reproduction by cell division: (binary fission/mitosis or meiosis).
Use of enzymes and other proteins coded for by DNA genes and made via messenger RNA intermediates and ribosomes.
Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is released and then used in metabolic pathways.
Response to external and internal stimuli such as changes in temperature, pH or levels of nutrients.
Cell contents are contained within a cell surface membrane that is made from a lipid bilayer with proteins embedded in it.
Some prokaryotic cells contain important internal membrane-bound compartments,[6] but eukaryotic cells have a specialized set of internal membrane compartments.
[edit] Anatomy of cells
There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.
[edit] Prokaryotic cells
Main article: Prokaryote
Diagram of a typical prokaryotic cellThe prokaryote cell is simpler than a eukaryote cell, lacking a nucleus and most of the other organelles of eukaryotes. There are two kinds of prokaryotes: bacteria and archaea; these share a similar overall structure.
A prokaryotic cell has three architectural regions:
on the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells;
enclosing the cell is the cell envelope – generally consisting of a cell wall covering a plasma membrane though some bacteria also have a further covering layer called a capsule. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. Though most prokaryotes have a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea)). The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and finally bursting (cytolysis) from osmotic pressure against a hypotonic environment. Some eukaryote cells (plant cells and fungi cells) also have a cell wall;
inside the cell is the cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions. A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Though not forming a nucleus, the DNA is condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids enable additional functions, such as antibiotic resistance.
[edit] Eukaryotic cells
Main article: Eukaryote
Diagram of a typical animal (eukaryotic) cell, showing subcellular components.
Organelles:
(1) nucleolus
(2) nucleus
(3) ribosome
(4) vesicle
(5) rough endoplasmic reticulum (ER)
(6) Golgi apparatus
(7) Cytoskeleton
(8) smooth endoplasmic reticulum
(9) mitochondria
(10) vacuole
(11) cytoplasm
(12) lysosome
(13) centrioles within centrosomeEukaryotic cells are about 15 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a cell nucleus, a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus." Other differences include:
The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.
Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be "viewed as sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."[7]
Eukaryotes can move using motile cilia or flagella. The flagella are more complex than those of prokaryotes.
Table 1: Comparison of features of prokaryotic and eukaryotic cells Prokaryotes Eukaryotes
Typical organisms bacteria, archaea protists, fungi, plants, animals
Typical size ~ 1–10 µm ~ 10–100 µm (sperm cells, apart from the tail, are smaller)
Type of nucleus nucleoid region; no real nucleus real nucleus with double membrane
DNA circular (usually) linear molecules (chromosomes) with histone proteins
RNA-/protein-synthesis coupled in cytoplasm RNA-synthesis inside the nucleus
protein synthesis in cytoplasm
Ribosomes 50S+30S 60S+40S
Cytoplasmatic structure very few structures highly structured by endomembranes and a cytoskeleton
Cell movement flagella made of flagellin flagella and cilia containing microtubules; lamellipodia and filopodia containing actin
Mitochondria none one to several thousand (though some lack mitochondria)
Chloroplasts none in algae and plants
Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells
Cell division Binary fission (simple division) Mitosis (fission or budding)
Meiosis
Table 2: Comparison of structures between animal and plant cells Typical animal cell Typical plant cell
Organelles Nucleus
Nucleolus (within nucleus)
Rough endoplasmic reticulum (ER)
Smooth ER
Ribosomes
Cytoskeleton
Golgi apparatus
Cytoplasm
Mitochondria
Vesicles
Lysosomes
Centrosome
Centrioles
Vacuoles
Nucleus
Nucleolus (within nucleus)
Rough ER
Smooth ER
Ribosomes
Cytoskeleton
Golgi apparatus (dictiosomes)
Cytoplasm
Mitochondria
[edit] Subcellular components
The cells of eukaryotes (left) and prokaryotes (right)All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, separates its interior from its environment, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell, then briefly describe their function.
[edit] Cell membrane: A cell's defining boundary
Main article: Cell membrane
The cytoplasm of a cell is surrounded by a cell membrane or plasma membrane. The plasma membrane in plants and prokaryotes is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of lipids (hydrophobic fat-like molecules) and hydrophilic phosphorus molecules. Hence, the layer is called a phospholipid bilayer. It may also be called a fluid mosaic membrane. Embedded within this membrane is a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is said to be 'semi-permeable', in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.
[edit] Cytoskeleton: A cell's scaffold
Main article: Cytoskeleton
Bovine Pulmonary Artery Endothelial cell: nuclei stained blue, mitochondria stained red, and F-actin, an important component in microfilaments, stained green. Cell imaged on a fluorescent microscope.The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division; and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microfilaments, intermediate filaments and microtubules. There is a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis.[8]
[edit] Genetic material
Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Most organisms use DNA for their long-term information storage, but some viruses (e.g., retroviruses) have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA) in organisms that use DNA for the genetic code itself. Transfer RNA (tRNA) molecules are used to add specific amino acids during the process of protein translation.
Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial chromosome) in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).
A human cell has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 23 pairs of linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production as well as specific tRNAs.
Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.
[edit] Organelles
Main article: Organelle
The human body contains many different organs, such as the heart, lung, and kidney, with each organ performing a different function. Cells also have a set of "little organs," called organelles, that are adapted and/or specialized for carrying out one or more vital functions.
There are several types of organelles within an animal cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.
Mitochondria and Chloroplasts – the power generators
Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell's energy by the process of oxidative phosphorylation, utilizing oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by splitting in two.
Organelles that are modified chloroplasts are broadly called plastids, and are involved in energy storage through the process of photosynthesis, which utilizes solar energy to generate carbohydrates and oxygen from carbon dioxide and water.[citation needed]
Mitochondria and chloroplasts each contain their own genome, which is separate and distinct from the nuclear genome of a cell. Both of these organelles contain this DNA in circular plasmids, much like prokaryotic cells, strongly supporting the evolutionary theory of endosymbiosis; since these organelles contain their own genomes and have other similarities to prokaryotes, they are thought to have developed through a symbiotic relationship after being engulfed by a primitive cell.[citation needed]
Ribosomes
The ribosome is a large complex of RNA and protein molecules. They each consist of two subunits, and act as an assembly line where mRNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes).[9]
Cell nucleus – a cell's information center
The cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical in shape and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm.
Diagram of a cell nucleus
Endoplasmic reticulum – eukaryotes only
The endoplasmic reticulum (ER) is the transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER plays a role in calcium sequestration and release.
Golgi apparatus – eukaryotes only
The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell. It is particularly important in the processing of proteins for secretion. The Golgi apparatus forms a part of the endomembrane system of eukaryotic cells. Vesicles that enter the Golgi apparatus are processed in a cis to trans direction, meaning they coalesce on the cis side of the apparatus and after processing pinch off on the opposite (trans) side to form a new vesicle in the animal cell.[citation needed]
Diagram of an endomembrane system
Lysosomes and Peroxisomes – eukaryotes only
Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. These organelles are often called a "suicide bag" because of their ability to detonate and destroy the cell.[citation needed]
Centrosome – the cytoskeleton organiser
The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.[citation needed]
Vacuoles
Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which are able to pump water out of the cell if there is too much water.
[edit] Structures outside the cell wall
[edit] Capsule
A gelatinous capsule is present in some bacteria outside the cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as bacillus anthracis or hyaluronic acid as in streptococci.[citation needed] Capsules not marked by ordinary stain and can detected by special stain. The capsule is antigenic. The capsule has antiphagocytic function so it determines the virulence of many bacteria. It also plays a role in attachment of the organism to mucous membranes.[citation needed]
